(1) Field of the Invention
This invention relates to an optical image reader and more particularly to improvements in and concerning an optical image reader which effects the reading of an image recorded on a given manuscript by projecting beams of light on the manuscript, receiving the reflected beams of light in a plurality of optical fibers and discerning the image based on the amount of light received in the plurality of optical fibers.
(2) Description of the Prior Art
The conventional optical image readers of the type under discussion are, for example, constructed as follows.
As illustrated in the drawing (FIG. 5), a plurality of light emitting side optical fibers T1-T4 respectively provided on one side with light emitting elements L1-L4 project beams of light on a given manuscript bearing an image at respective read points D1-D4, a plurality of light receiving side optical fibers R1-R4 disposed correspondingly to the optical fibers T1-T4 admit the beams of light reflected from the read points D1-D4, a photoelectric conversion element P converts the beams of light received in the light receiving side optical fibers R1-R4 into electric signals, a DC amplifier A1 amplifies the electric signals and feeds out the amplified electric signals, a comparator D1 receives the detection signals SG1 issuing from the DC amplifier A1, compares the levels of the detection signals SG1 originating in the read points D1-D4 with the reference level h1 set in advance, converts the results of the comparison into binary digital signals, feeds the digital signals to a microcomputer M1 and the microcomputer M1 then reads the image informations at the respective read points D1-D4 based on the digital signals so received.
In the optical image reader constructed as described above, the output waveforms of the detection signals SG1 issuing from the DC amplifier A1 have levels corresponding to the degrees of the shade of the image information at the read points D1-D4 as shown in the drawing, (FIG. 6). These detection signals SG1 are fed out to the comparator D1 at the next stage. The comparator D1 compares these detection signals SG1 with the reference level h1 and converts the individual signals into binary digital signals based on the rule that each signal smaller than the reference level h1 is designated by "0" and each signal larger than the reference level h1 by "1". The resulting set of digital signals are fed out to the microcomputer M1, which reads the images based on these digital signals.
Incidentally, the aforementioned DC amplifier A1 by nature is prone to degradation of its operational characteristics due to the influence of aging, temperature, etc. It is, therefore, not impossible that even when an electric signal brought in from the photoelectric conversion element P has the level of OV, the DC amplifier A1 produces at its output terminal an offset voltage .DELTA.V corresponding to the change in the aforementioned, characteristics. When the offset voltage .DELTA.V is generated, the individual detection signals SG1 from the DC amplifier A1 have levels including the offset voltage .DELTA.V [See FIG. 6 (b)]. If this offset voltage .DELTA.V varies from time to time, the comparator D1 at the next stage is barred from producing digital signals accurately corresponding to the image.
It has been customary to cope with the adverse effects of this random variation of the offset voltage .DELTA.V by providing the DC amplifier A1 with an offset adjusting circuit B1 which is adapted to adjust the offset voltage. This method which thus relies for the necessary adjustment upon the offset adjusting circuit B1 is effective, though not permanently, in curbing the influence of the offset voltage .DELTA.V and enabling the optical image reader to read the image accurately. When the offset voltage .DELTA.V is caused to vary gradually due to a change of temperature, change due to aging, change of brightness in the area surrounding the image reading head, etc., however, there ensues the problem that the method fails to provide adjustment accurately following the gradual variation.